20 stickers are in 3 boxes.
The 1st box has 2 times as many stickers as the 2nd box.
The 3rd box is 7 times as many stickers as the 2nd box.
How many less stickers are there in the 2nd box than in the 1st box and 3rd box?
Choose the box with the least stickers to be 1 u.

Number of stickers in the 2nd box = 1 u
Number of stickers in the 1st box = 2 x 1 u = 2 u
Number of stickers in the 3rd box = 7 x 1 u = 7 u

Total number of stickers
= 1 u + 2 u + 7 u
= 10 u

10 u = 20
1 u = 20 ÷ 10 = 2

Number of stickers less in the 2nd box than in the 1st and 3rd boxes
= Total number of stickers in 1st and 3rd box - Number of stickers in 2nd box
= 2 u + 7 u - 1 u
= 8 u 
= 8 x 2
= 16

Answer(s): 16